Researchers have used lifecycle assessment, which provides a more complete …

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Passenger transportation accounts for roughly 20 percent of the US' annual energy use, but it tends to get considered in only the most simplistic terms: miles per gallon, the figure that will be used to calculate the US fleet's new efficiency standards. But cars don't simply appear, ready to drive, in their owner's garage, and building the road and service infrastructure that makes them a useful commodity can add significant energy costs. The field of lifecycle assessment was developed to account additional factors like these, and researchers have now used it to tackle the energy efficiency and pollutant production by the US passenger transit system. The results emphasize the role of infrastructure, and highlight how secondary factors can make a huge difference.

The paper that describes the work is open access, so anyone can have a look at some of the details. In short, researchers compared a variety of hardware within a number of key segments: automobile, rail, and air transit. So, for example, the automotive analysis compared a Toyota Camry, a Chevy Trailblazer, and a Ford F-150, which provide a reasonable cross-section of the vehicles typically in use. A similar spread was used for aircraft, and different transit systems in Boston and the Bay Area of California were used for rail. The full lifecycle considered included everything from fuel and vehicle production to more obscure factors, such as the energy used in lighting parking lots and that used in insurance and maintenance.

The basic calculations show that, when expressed in terms of MegaJoules per passenger-kilometers traveled (MJ/PKT), different forms of transportation had very different sensitivities to external factors, and there were often substantial differences within a category. So, for example, a diesel bus running during peak hours blew away its automotive competition, coming in at just over 0.5 MJ/PKT, while the most efficient vehicle, the Camry, was just under 3 MJ/PKT, and the pickup truck nearly at five. Switch to off-peak, however, and the nearly empty bus will be just as bad as the pickup.

For the most part, automotive transit, with the exception of off-peak-hour busses, performed the worst, and about 70 percent of their energy costs came directly from running the vehicle. The same fraction held for planes, but they manage to go much further on the energy they burn, and their infrastructure footprint is tiny compared to a car's. As such, the large aircraft (a 747) used for the comparisons was roughly as efficient as light rail. Rail systems were by far the most efficient, but nowhere near as good as their fuel efficiency might suggest, as roughly 70 percent of their energy use came from building and maintaining infrastructure.

The authors also evaluated a number of relevant pollutants. It's no surprise that, to a very large extent, energy use and CO2 emissions were roughly equivalent. There was one notable exception here: San Francisco's Muni isn't as energy efficient as Boston's Green Line, but it gets a lot of its power from hydroelectric sources, so it ends up beating the Green Line in this regard. Given California's high standards for renewable energy, it's likely to expand upon that lead.

Regulations played a huge role for other pollutants. Sulfur has largely been eliminated from vehicular fuels, meaning that infrastructure and manufacturing costs dominated these numbers—except in the cases where the primary power is electric, where coal-based power generation pushed the Green Line to the top. Nitrogen emissions aren't regulated nearly as tightly, so the off-peak bus wound up producing really ugly numbers in this analysis.

The analysis definitely has room for improvement. For example, the authors don't have hard numbers for the percentage of recycled materials used in either vehicle or infrastructure production, so they assume everything is built from raw materials up. Still, the numbers drive a couple of things home. The first is that passenger occupancy plays a huge role in driving these numbers; the authors note that just adding a passenger to an SUV makes it equivalent to a bus carrying eight passengers. As a result, it may be possible to change the efficiency of an existing system simply through measures that encourage carpooling or public transit use.

The other notable aspect of these numbers is how they likely represent a moving target. Plug-in hybrids are likely to start arriving on the US market within the next few years, shifting some of the automotive emissions to large-scale power generation. Meanwhile, many states have set targets for renewable energy that will lower the emissions associated with that energy production. The authors also note that biofuels may complicate the analysis, with individual sources having very different energy profiles.

All of that means that this report shouldn't be viewed as the final word. Instead, the analysis should help policy makers identify the low-hanging fruit, efficiency-wise, while the data and methods should provide a foundation for them to analyze different courses of action and track the impact of changes to the US transportation system.